1. Field of the Invention
The present invention relates to an optical transmission apparatus and a method of controlling the same, and in particular, to an optical transmission apparatus that is suitably used in an optical network for multiwavelength transmission of optical signals, and a method of controlling the same.
2. Description of the Related Art
In recent years, as one example of multiwavelength optical transmission systems, an optical network (metro core) system has been gaining attentions in which system local cities as regional bases are connected and an optical signal with a given wavelength may be added or dropped at a given node provided for a base.
FIG. 6 is a block diagram showing a configuration example of a metro core system. In a multiwavelength optical transmission system 100 that constitutes the metro core system, a plurality of OADM (Optical Add-Drop Multiplexer) nodes 101 (for example, 101-1, 101-2, and 101-n in FIG. 6) capable of adding or dropping optical signals, an optical broadband HUB that is not depicted in the drawing, an optical switching HUB, and the like are connected through a transmission line optical fiber 102, respectively. Further, by setting a signal path between given OADM nodes 101 in the plurality of OADM node 101, communication by means of multiwavelength signal light becomes possible.
It should be noted that optical amplifiers 103 (preamplifier and postamplifier) are provided, as necessary, respectively at stages preceding and following each of the OADM nodes 101 and the optical transmission apparatus. These amplifiers serves to compensate loss of signal light power received at the transmission line optical fiber 102 and the OADM node 101.
In the communication between the above OADM nodes 101, because switching between signal paths is dynamically performed at the OADM node 101 in operation, a number of signal wavelengths transmitted through the system dynamically varies. It is necessary to maintain communication quality of individual optical signals even if such a variance in the number of signal wavelengths occurs frequently. In order to maintain output optical power of each signal wavelength constant against this variance in the number of wavelengths (maintain gain flatness to wavelength), an AGC amplifier having an automatic gain control (AGC) function is typically employed for the optical amplifier 103. A fiber amplifier such as an EDFA (Erbium Doped Fiber Amplifier), for example, is used as this optical amplifier 103.
FIG. 7 is a diagram showing a configuration example of the OADM node 101. This OADM node 101 is provided with a wavelength demultiplexer 111, a branch coupler 112 for branching the signal light at its node 101 (to a drop path 101d), an optical switch 113 for selecting either to pass the signal light or add (light from an add path 101a) at its node 101, a Variable Optical Attenuator (VOA) 114 for adjusting power of the light from the optical switch 113, and a wavelength multiplexer 115.
An attention is now paid to a configuration of an OADM node 101-2 shown in FIG. 6. Multiwavelength signal light input from an OADM node 101-1 that is provided upstream to the OADM node 101-2 in the transmission line optical fiber 102 is divided by the wavelength demultiplexer 111 included in the OADM node 101-2 into individual signal wavelengths, as shown in FIG. 7.
Then, it is determined whether each divided signal light is dropped at the OADM node 101-2 or passed (through) according to the setting of the optical switch 113 that is positioned next. For the wavelength that is dropped at the OADM node 101-2, the light from the add path may be added to the light output to the side of the OADM node 101-n by the optical switch 113. In this manner, the output power of the signal light that has passed the optical switch 113 is adjusted at the VOA 114 to be output to the wavelength multiplexer 115. At the wavelength multiplexer 115, the optical signals divided into the individual wavelengths are re-multiplexed, and output to the side of the OADM node 101-n through the transmission line optical fiber 102 that is on the downstream.
FIG. 8 is a diagram showing an example of spectra of the multiwavelength light transmitted through the transmission line optical fiber 102 in the communication between the OADM nodes 101; in other words, the diagram shows an example of the light spectra output from the wavelength multiplexer 115 shown in FIG. 7. An abscissa axis indicates the wavelength (λ), and an ordinate axis indicates the light power (Power). At the wavelength multiplexer 115, as shown in FIG. 8, the signal light of each wavelength (see A in FIG. 8), and spontaneous emission light having a wavelength range of the signal light as noise light produced in amplification at the optical amplifier 103 (ASE: Amplified Spontaneous Emission, see B in FIG. 8) are transmitted in an accumulated state to the transmission line optical fiber 102 on the downstream side.
Note that, a path for a wavelength that is unused as signal light (see C in FIG. 8) between the OADM nodes 101, it is set so that the light does not pass based either on the switching at the optical switch 113 or on light attenuation control at the VOA 114.
Note that, Patent Document 1 describes a transmission apparatus that is provided with a VOA equivalent to that of the conventional art as described above.
Further, as the conventional art related to the present invention, there is a transmission apparatus as described in Patent Document 2.
[Patent Document 1] Japanese Patent Laid-Open No. 2003-163641
[Patent Document 2] Japanese Patent Laid-Open No. 2002-353939
However, in the conventional optical transmission apparatus, as shown in FIG. 6, if a number of transmission wavelengths changes when the OADM nodes 101 in the multiwavelength transmission system 100 are in a cascading connection, a delay occurs transitionally in power control of optical signals immediately after the change. Specifically, immediately after the number of transmission wavelengths changes, in a state that VOA control does not transitionally reflect the change of the number of transmission wavelengths, the optical signals are output to the downstream-side transmission line.
Therefore, a problem has been noted that, when performing communication between the OADM nodes provided at multiple stages, it is expected that an amount of variation in the signal power is accumulated, and accordingly, the power of the optical signals in a receiving-side apparatus goes down below a reception range, i.e. the signals may not be recognized as signals or a signal-to-noise ratio deteriorates, thereby causing a reception error or deterioration in communication quality.
FIG. 9 to FIG. 11 are illustrations of examples of signal power change according to the change in the number of signal wavelengths. A case is assumed in which, as shown in FIG. 9, for example, when one wavelength (λ1) is added from the OADM node 101-1 and one wavelength (λ2) is added from the OADM node 101-2 in the multiwavelength transmission system 100, an interruption occurred in the OADM node 101-1 eliminates the one wavelength transmitted from the OADM node 101-1, thereby reducing the number of signal wavelengths to optical signals of only one wavelength (λ2) from optical signals of two wavelengths (λ1, λ2).
FIG. 10(a) and FIG. 10(b) respectively show a change in wavelength in the stage for being output from the wavelength multiplexer 115 in the OADM node 101-2 when the optical signals change from two wavelengths to one wavelength in this manner. An abscissa axis indicates wavelength (λ), and an ordinate axis indicates power of each optical signal.
At this time, at the preceding stage at which the number of signal wavelengths decreases, as shown in FIG. 10(a), light including the optical signals (λ1, λ2) and ASE (λ1_ase, λ2_ase) in the vicinity of the optical signals (λ1, λ2) is output as output light output from the wavelength multiplexer 115. The reason why ASE including a wavelength component that is not in the vicinity of the optical signals (λ1, λ2) is not output is that the ASE outside the wavelength range that is used for the optical signals is eliminated by the operation of the optical switch 113 or the VOA 114.
In contrast, after the number of wavelengths decreases in the above manner, the one-wavelength optical signals are input to the OADM node 101-2. At this time, immediately after the change in the number of signal wavelengths, until the VOA 114 in the OADM node 101-2 detects the absence of signals and performs adjustment, when routed through the wavelength demultiplexer 111, as shown in FIG. 10(b), the signal light λ2 and ASE (λ1_ase, λ2_ase) respectively in the vicinity of the wavelength λ1, λ2 are output to be input to the following optical amplifier 103.
Immediately after the number of wavelengths of the optical signals becomes one wavelength, in order to maintain the communication quality of the optical signals, the following optical amplifier 103 automatically controls gain of the signal light λ2 so that the signal light λ2 is stabilized to an average power of the wavelength components (λ1, λ2). Specifically, a target power at which the gain of the signal light λ2 is controlled becomes equal to an average value of the power of the signal light λ2 at this time and the power of the ASE light component λ1_ase after the signal light component is removed to be relatively low in level.
Further, when such a change in the number of wavelengths occurs, there is a case in which the change in the signal light gain increases with SHB (spatial hole burning) being a major factor, for example, during a transitional period of time until which response control by the AGC amplifier 103 is stabilized. FIG. 11(a) and FIG. 11(b) are diagrams showing light spectra output from the optical amplifier 103 in the change of the signal wavelengths shown in FIG. 9. An abscissa axis indicates wavelength (λ) and an ordinate axis indicates power of each optical signal.
At this time, in a state before the change of the number of signal wavelengths, as shown in FIG. 11(a), because the gain of the optical signals λ1 decreases when the wavelength of the optical signals λ1 input into the optical amplifier 103 corresponds to a hole of the SHB, a power deviation virtually occurs between the two optical signals λ1 and λ2 output from the optical amplifier 103.
After this, when the number of signal wavelengths changes to eliminate the one wavelength (λ1) on a short wavelength side, and only the one wavelength (λ2) on long wavelength side is left, an effect of the SHB is reduced, the optical amplifier 103 reduces the power that the one wavelength (λ2) originally had by an arrow Pd, during the transitional response time period immediately after the change of the number of signal wavelengths, as shown in FIG. 11(b).
Such power, that is, the amount of gain variation may be ignorable as a slight variation in the amount of power when considering the single OADM node 101-2 alone. However, when the OADM nodes 101-1 to 101-n are in a cascading connection, the variation in signal power is eventually accumulated as moving onto a downstream-side stage in the transmission path. A problem has been noted that, when an amount of the accumulation of the variation in signal power becomes large, the power of optical signals goes down below a reception range, i.e. the signals may not be recognized as signals or a signal-to-noise ratio deteriorates, thereby causing a reception error or deterioration in communication quality.
Examples of factors that cause the signal gain variation in response to the change in the number of wavelengths include (1) SHB as described above, as well as (2) gain (wavelength) deviation and (3) Stimulated Raman Scattering (SRS) effect. The following describes these factors, respectively.    (1) SHB
A first factor, the SHB, is a phenomenon caused when EDFA is applied to the optical amplifier 103, for example, and shows a characteristic that the optical signal gain on the short wavelength side becomes lower. For example, when optical signals of one wavelength (for example, 1538 nm) in a C band (1530-1565 nm) is input to the optical amplifier 103, phenomena are caused such that an EDFA gain in the vicinity of the signal wavelength is decreased (this phenomenon is called a main hole), and an EDFA gain in the vicinity of 1530 nm is also decreased (this phenomenon is called a second hole).
Further, the SHB has a characteristic that the main hole in the C band becomes deeper on the short wavelength side (larger in an amount of gain decrease), and both the main hole and the second hole becomes deeper as the optical signal input power is greater. In addition, the SHB has a smaller effect when signal light of multiple wavelengths is input, and its effect increases as the number of wavelengths that are input decreases.    (2) Gain Deviation
A second factor, the gain (wavelength) deviation, is a phenomenon caused in an optical amplifier. Specifically, the optical amplifier controls so that an average gain of the signal light is maintained constant (AGC), and when a wavelength having a deviation remains, the output optical power of the remaining optical signals varies because the optical amplifier operates so that the gain of the signal light matches with the target gain.    (3) SRS Effect
A third factor, the SRS effect, is a phenomenon caused in the optical fiber transmission line or the distributed compensation fiber. In transmitting the multiwavelength optical signals through the optical fiber, the signal light power on the short wavelength side amplifies signal light on the long wavelength side as excitation light power, and as a result, a phenomenon is caused such that the signal light power becomes larger on the long wavelength side. Therefore, when the signal on the short wavelength side is eliminated, the remaining signal light on the long wavelength side cannot obtain power from the short wavelength side, thereby causing the power decrease.
As described above, when the number of wavelengths of the transmitted multiwavelength signal light changes, mainly due to the three factors of the SHB, the gain deviation, and the SRS, the signal light power of the remaining signal light (remaining channel) varies. Even if the variation per span is not significantly large, the variation in light power of each signal wavelength produced in each of the optical amplifiers and the optical fiber transmission lines is accumulated in a long distance system provided with a number of stages of optical amplifiers (see reference number 103 in FIG. 6) that respectively perform AGC.
With an optical transmission system like the conventional art in which the transmission distance is short and only a smaller number of optical amplifiers are provided, such a variation is indifferent and does not cause a problem. However, in the future, an increasing number of optical amplifiers along with the increase in the distance of the system may cause the optical signals power on the reception end to exceed the tolerance level of reception, resulting in a cause of transmission error.
The technique described in Patent Document 2 does not provide any solution for such a problem.